One class of opto-electrical devices is that using an organic material for light emission or detection. The basic structure of these devices is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline)aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent anode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium.
In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light.
These devices have great potential for displays. However, there are several significant problems. One is to make the device efficient, particularly as measured by its external power efficiency and its external quantum efficiency. Another is to optimise (e.g. to reduce) the voltage at which peak efficiency is obtained. Another is to stabilise the voltage characteristics of the device over time. Another is to increase the lifetime of the device.
To this end, numerous modifications have been made to the basic device structure described above in order to solve one or more of these problems.
One such modification is the provision of a layer of conductive polymer between the light-emissive organic layer and one of the electrodes. It has been found that the provision of such a conductive polymer layer can improve the turn-on voltage, the brightness of the device at low voltage, the efficiency, the lifetime and the stability of the device. In order to achieve these benefits these conductive polymer layers typically may have a sheet resistance less than 106 Ohms/square, the conductivity being controllable by doping of the polymer layer. It may be advantageous in some device arrangements to not have too high a conductivity. For example, if a plurality of electrodes are provided in a device but only one continuous layer of conductive polymer extending over all the electrodes, then too high a conductivity can lead to lateral conduction and shorting between electrodes.
The conductive polymer layer may also be selected to have a suitable workfunction so as to aid in hole or electron injection and/or to block holes or electrons. There are thus two key electrical features: the overall conductivity of the polymer composition; and the workfunction of the polymer composition. The stability of the composition and reactivity with other components in a device will also be critical in providing an acceptable lifetime for a practical device. The processability of the composition will be critical for ease of manufacture.
One example of a suitable conductive polymer for use as a hole injection layer between the anode and the light-emissive organic layer is polystyrene sulphonic acid doped polyethylene dioxythiophene (“PEDOT-PSS”)—see EP 0,686,662. This composition provides an intermediate ionisation potential (intermediate between the ionisation potential of the anode and that of the emitter) a little above 4.8 eV, which helps the holes injected from the anode to reach the HOMO level of a material, such as an organic light emissive material or hole transporting material, in an adjacent layer of an opto-electrical device. The PEDOT-PSS may also contain epoxy-silane to produce cross-linking so as to provide a more robust layer. Typically the thickness of the PEDOT/PSS layer in a device is around 50 nm. The conductance of the layer is dependent on the thickness of the layer.
The chemical structures for PEDOT and PSS are shown in formulae 1 to 4 below:

In a PEDOT-PSS composition the PEDOT oxidises to produce a polymer radical cation which acts as a hole transporter. The PSS ionises to produce a polymer anion which acts as a counter ion to stabilise the charge on the PEDOT. Previously small counter ions had been used for stabilizing the charge on the PEDOT. However, it was found that small counter ions migrated though the system when subjected to an electromagnetic field causing charge separation and charge build up in localized areas/interfaces of a device resulting in poor performance. Migration of the counter ion can also lead to adverse reactions with materials in adjacent layers. Larger counter ions, for example polymeric counter ions such as PSS, are found to be advantageous as they do not diffuse through an opto-electronic device when the device is switched on.
PEDOT:PSS is water soluble and therefore solution processible. The provision of PEDOT:PSS between an ITO anode and an emissive layer increases hole injection from the ITO to the emissive layer, planarises the ITO anode surface, preventing local shorting currents and effectively makes energy difference for charge injection the same across the surface of the anode.
It has been found that varying the ratio of PEDOT:PSS in a layer of a device significantly changes the functional performance of the device.
A PEDOT:PSS ratio of 1:2.5 provides a stable processible solution. That is, materials with this ratio or higher PSS stay in solution. At low concentrations they come out of solution. Without being bound by theory, it is thought that the PEDOT radical cations are stabilized in solution as a result of the sulphonate counter ions surrounding the PEDOT radical cations and forming micel type structures. The sulphonate counter ions are more hydrophilic than the PEDOT radical cations and aid in forming a suspension of the PEDOT radical cations. As a result longer chain (higher molecular weight) PEDOT molecules can be formed during polymerisation without falling out of solution. Further, longer chain PEDOT molecules are easier to oxidize and produce better hole transport. Therefore, the sulphonate counterions aid hole transport by stabilising the PEDOT radical cations. However, at a ratio of 1:2.5 the conductivity is very high and as such this material cannot be used in some opto-electronic device arrangements as it can, for example, short connections between electrode lines in a device as discussed previously.
In practice, it has been found that using an excess of PSS can improve device performance and, in particular, can increase lifetime. Furthermore, excess PSS results in the composition being easier to ink jet print. By “excess PSS” is meant more PSS than is needed to prevent the PEDOT falling out of solution. Thus, using excess PSS, such as a PEDOT:PSS ratio of 1:20 is useful in working devices. Without wishing to be bound by theory the present inventors propose several explanations for the improved device performance that is observed when using an excess of PSS. The first of these relates to conductivity.
The PSS content affects the conductivity of the composition. In this regard, the present applicants consider that there are two types of conduction important for providing good hole transport: ionic conduction (e.g. by H+ ions); and radical cation (hole) conduction (e.g. via PEDOT). Without wishing to be bound by theory, the applicants believe that the excess PSS contributes to both types of conduction. It appears that the hydrogen ions contribute to the conductivity of the composition via ionic conduction. Further, as discussed above, the sulphonate stabilizes the PEDOT radical cation to aid hole transport. The former effect (ionic conductivity) will be dependent on the amount of PSS, with a large excess increasing ionic conductivity. The latter effect (hole conductivity) will not be as sensitive to the amount of PSS present as the effect will saturate when sufficient PSS is present to stabilize the radical cation.
Another possible mechanism to explain the improvement in device performance when using excess PSS is that the PSS is more hydrophilic than the PEDOT. Accordingly, excess PSS increases film uniformity with an adjacent polymer layer as the excess PSS results in the composition being more hydrophilic causing less mixing with the adjacent polymer layer.
It is evident from the above that it is advantageous to provide PSS in excess for ease of manufacture of a device and so as to produce a device with better performance and lifetime. However, there is always a desire to further improve the performance and lifetime of devices and make the manufacturing process easier and cheaper. Accordingly, alternatives to the PEDOT-PSS system having excess PSS are sort. Without being bound by theory, one possible limitation on the lifetime of devices using the aforementioned PEDOT-PSS system is that the provision of such a large excess of PSS results in a composition which is very acidic. This may cause several problems. For example, providing a high concentration of strong acid in contact with ITO may cause etching of the ITO with the release of indium, tin and oxygen components into the PEDOT which degrades the overlying light emitting polymer. Furthermore, the acid may interact with light emitting polymers resulting in charge separation which is detrimental to device performance.
Since PEDOT:PSS is solution processible, it is desirable to also enable the deposition of PEDOT:PSS and alternatives thereof according to the present invention using ink jet printing techniques. The key reasons for the interest in ink jet printing are scalability and adaptability. The former allows arbitrarily large sized substrates to be patterned and the latter should mean that there are negligible tooling costs associated with changing from one product to another since the image of dots printed on a substrate is defined by software.
The deposition of material for organic light emitting diodes (OLEDs) using ink jet printing techniques are described in a number of documents including, for example: T. R. Hebner, C. C. Wu, D. Marcy, M. H. Lu and J. C. Sturm, “Ink-jet Printing of doped Polymers for Organic Light Emitting Devices”, Applied Physics Letters, Vol. 72, No. 5, pp. 519-521, 1998; Y. Yang, “Review of Recent Progress on Polymer Electroluminescent Devices,” SPIE Photonics West: Optoelectronics '98, Conf. 3279, San Jose, Jan., 1998; EP O 880 303; and “Ink-Jet Printing of Polymer Light-Emitting Devices”, Paul C. Duineveld, Margreet M. de Kok, Michael Buechel, Aad H. Sempel, Kees A. H. Mutsaers, Peter van de Weijer, Ivo G. J. Camps, Ton J. M. van den Biggelaar, Jan-Eric J. M. Rubingh and Eliav I. Haskal, Organic Light-Emitting Materials and Devices V, Zakya H. Kafafi, Editor, Proceedings of SPIE Vol. 4464 (2002). Ink jet techniques can be used to deposit materials for both small molecule and polymer LEDs.
A volatile solvent is generally employed to deposit organic electronic material, with 0.5% to 4% dissolved material. This can take anything between a few seconds and a few minutes to dry and results in a relatively thin film in comparison with the initial “ink” volume. Often multiple drops are deposited, preferably before drying begins, to provide sufficient thickness of dry material. Precision ink jet printers such as machines from Litrex Corporation of California, USA are used; suitable print heads are available from Xaar of Cambridge, UK and Spectra, Inc. of NH, USA.
Accordingly, there is a desire to provide an alternative to the aforementioned system, preferably one which results in better device performance, lifetime and ease of manufacture.
It is an aim of the present invention is to solve one or more of the problems outlined above.